1. Field of the Invention
This invention pertains generally to detection and annunciation apparatus and, more particularly, to such an apparatus for detecting and annunciating arc faults. The invention also relates to methods and systems for detecting and annunciating arc faults.
2. Background Information
Arcing is a luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of electrodes. An arc fault is an unintentional arcing condition in an electrical circuit, such as an electrical conductor. Arc faults can be caused, for instance, by worn insulation between adjacent bared conductors, by other insulation failures and in other situations where conducting elements are in close proximity. Arc faults in systems can be intermittent since the magnetic repulsion forces generated by the arc current force the conductors apart to extinguish the arc. Mechanical forces then bring the conductors together again in order that another arc is struck. Arcing faults between two conductors can also result from an over the surface conductive path forming on an insulator caused by, for example, salt water. Such a fault can be intermittent, since the resulting heat will dry the water and terminate the arcing condition. This process can then repeat.
During sporadic arc fault conditions, the overload capability of a circuit breaker will not function since the root-mean-squared (RMS) value of series arc fault current pulses is too small to activate the thermal based automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker adds one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
A cast-iron manhole cover can weigh between about 85 and 300 pounds (about 39 to 136 kg). Explosions have propelled these massive discs anywhere from 1 foot to 50 feet (about 0.3 to 15 m) into the air. These explosions cause a loss of power in the aftermath and, moreover, provide a risk of injury. Such manhole explosions are typically caused when a spark from electrical wiring ignites gas inside the manhole. For example, underground power cables can become frayed from aging, from corrosive chemicals (e.g., road salt), from overload or from vermin biting them. Such cables may carry, for example, 120/208 VAC or 13 KVAC. Under arc fault conditions, in which the arcing current is believed to be relatively small as compared to the normal power line current, the arcing heats the insulation of the cables. Fuses called “limiters” in this application protect the cables. The limiters are designed to prevent long term overheating of the cable's insulation due to a sustained overcurrent condition. However, the limiters do not respond to an insulation related arc fault because the current's RMS value is too low.
It is believed that network relays cannot be employed as suitable protectors either, since the network relays cannot detect the relatively low level arcing current, much less identify the location of the arc fault. If a protector cannot locate the fault, even if it knew there was a fault, it would not be advisable to open the protector since a relatively large geographic area would be without power. Also, once the network is deenergized, the smoke from the fault would cease and, thus, the fault could not be located.
As the arcing heats the cable insulation, the insulation smolders and releases flammable gases. Further, hydrogen may be formed from a reaction of water and copper or aluminum with heat from parallel arcing. In turn, the pressure from the gases may build up inside the manhole. The arcing, also, may ignite the insulation and/or the gases, which can cause a powerful explosion. Depending on the amount of gas pressure built up and the gas fuel-to-air mixture inside the manhole, the cover may flip over or be launched several feet or more in the air. Alternatively, there may not be an explosion, but just a substantial amount of smoke or fire.
Accordingly, there is a substantial need to improve underground power cabling. For example, some power companies are in the process of replacing solid manhole covers with slotted manhole covers. These new covers allow the gas to be released less violently and, also, give an early warning to possible explosions. See, for example, http://science.howstuffworks.com/exploding-manhole1.htm. However, these slotted manhole covers cannot be used in locations where run off from a rainstorm could result in manhole flooding.
The time from initial arcing to smoke, fire and/or explosion is not known, but may be relatively long (e.g., months or years). Furthermore, such underground problems can go undetected until a smoking manhole is reported. Then, a crew must go to the site, remove the manhole cover and attempt to isolate the problem in the underground conduit by cutting power cables. This may take time and money and, moreover, may subject the crew to dangerous conditions.
Hence, there remains the very real and substantial need for a method and a system to reliably detect and communicate an arcing power cable in an underground conduit before it becomes an explosion hazard.
Accordingly, there is room for improvement in apparatus, methods and systems for detecting and annunciating arc faults.
It is known to employ a current transformer in a circuit breaker trip unit for sensing current and for powering the trip unit. Another means for sensing an alternating current (AC) current utilizes a Rogowski coil. The Rogowski coil includes a helical coil of wire with a lead from one end of the coil returning through the center of the coil to the other end, in order that both terminals are at the same end of the coil. The whole assembly is then wrapped around a straight conductor. A “split” core is one way of doing this. The resultant voltage induced in the coil is proportional to the rate of change of current in the straight conductor. Since the Rogowski coil has an air core, rather than an iron core, it has a relatively low inductance and can respond to relatively fast-changing currents. Also, because it has no iron core to saturate, it is highly linear even when subjected to relatively large currents, such as those used in electric power transmission and welding. See, for example, http://www.answers.com/topic/rogowski-coil.